Originally Posted by Flying Lawyer

It is not the owners responsibility, but the drivers responsibility. As it is only illegal to drive in winterly conditions without winter tires (and in some winters we so not see any of these in most parts of Germany) this even makes some sense (and more money for Hertz). The leave no doubt on their websites that they charge extra for winter tires (however, no need at all to tip the guy handing you over the keys - habits are different in different places).